Here's why:
* It contains a subject ("she") and a verb ("was"). This makes it a clause.
* It begins with a subordinating conjunction ("because"). This means it cannot stand alone as a complete sentence and needs an independent clause to make sense.
For example:
* She couldn't go to school because she was sick.
In this example, "Because she was sick" depends on the independent clause "She couldn't go to school" to form a complete sentence.